Noise in video signals falls mainly into two broad categories.
The first type is the noise generally referred to as the signal to noise ratio of the signal, and occurs more or less uniformly throughout the signal. In an audio system, this would be equivalent to the familiar background hiss.
In this disclosure "signal to noise ratio" type of noise will be referred to as "spectrum" noise, to distinguish it from the second type of noise, "impulse" noise.
Impulse noise affects mostly only small sections of a video signal, while surrounding portions of that video signal generally remain unaffected and "clean". In an audio system, this would be equivalent to static.
Impulse noise also tends to be more severe than spectrum noise, and although it is of short duration, it often totally destroys all frequencies of the video signal for the duration of its occurrence.
When noise enters a video system, the result is a deterioration of picture quality. Both types of noise cause a loss of information by distorting a portion of the desired video signal. Obviously, the less noise there is in a video system, the better.
Spectrum noise tends to deteriorate the high frequencies of the video signal that carry the fine detail picture information. A result of increasing spectrum noise is a kind of graininess of the resulting picture. As the spectrum noise increases further, the grain becomes more evident in the viewed picture.
To make matters worse, once the signal to noise ratio has been established at a low amplitude level, further amplification of the signal does not improve picture quality, since the noise is amplified right along with the wanted signal.
For this reason, simple field strength and video level meters, which can be purchased at a reasonable price, cannot properly evaluate the QUALITY of a video signal, only its strength.
A quick and economical way to evaluate and "grade" the quality of a video signal would be a good first step to improving the signal, by allowing the checking of video transmission and receiving equipment and cable lines. The present invention will also quickly detect and identify a substandard video tape, detect clogging video heads at a very early stage even before their effect is readily visible, allow a quick "red light-green light" indication of signal quality, assist in both the manual and automatic setting of the playback tracking control properly, and provide a full grading scale from poor to excellent. It works with tape as well as "live" video signals, and also lends itself very well to battery operation, to extend its benefits into "field" applications.
Spectrum noise evaluation is the essence of one embodiment of the present invention.
As to the matter of impulse noise, it mostly makes itself directly visible as light or dark spots and streaks in the viewed picture.
Impulse noise can become indirectly visible when it upsets processing equipment. It is possible for a single pulse of impulse noise lasting only microseconds to upset processing equipment so badly that a loss of picture lasting several seconds occurs. A classic example of this occurs when a single noise pulse upsets the scanning countdown timer in a television receiver, and the picture "rolls" vertically for several seconds before locking back in sync. Similarly, that same single impulse noise pulse can upset the servo locking in a video tape recorder, and cause the video heads to run uncontrolled and out of phase for several seconds before recovering.
The present invention will remove this noise before it can upset the equipment.
Video processing equipment is more likely to be disturbed by impulse noise than by spectrum noise not only because it is more severe, but also because video equipment is controlled more by the low frequency content signals that impulse noise destroys than it is by the high frequency content signals that spectrum noise attacks.
Severe playback tracking errors, defective recordings, and physically damaged video tapes generate impulse noise anywhere within the picture frame, disrupting video equipment processing this corrupted video signal.
If as a result of the signal evaluation that it makes, the invention determines that the video signal contains noise that would disrupt processing equipment, then that noise is automatically removed.
Impulse noise upsets televisions and video recorders, and equipment such as amplifiers, color correctors, and enhancers, as well as equipment that keeps track of frame timing by counting.
When impulse noise is negative going, and approaches the level of the normal horizontal sync pulses, it can cause the automatic gain control circuitry in amplifiers and video recorders to take additional agc readings of the video signal at abnormal times.
When the negative going noise pulses are directly followed by a level that is more positive than the normal back-porch level, they change the perceived horizontal back porch black level, causing processing equipment to falsely interpret this video signal as being larger then it really is, and automaticly reduce its gain to an abnormally low level.
The invention will detect this condition, and remove the noise.
Additionally, extra horizontal sync pulses simulating negative going impulse noise, and companion positive going pulses, have been intentionally added to some video recordings to cause video recorders to have the same kind of problems as described above, and thereby corrupt the recording of the video signal.
The invention will detect this kind of impulse noise also, and remove it.
For the remainder of this disclosure, to avoid duplicate discussion, except where referred to in the text, these intentionally added pulses will be considered together with, and considered as, unwanted impulse noise.
Because they are confined to the normally black area of the vertical interval, the intentionally added pulses can also cause additional problems with television receivers that are not operating quite properly, where the set owner is totally unaware of the existence of any malfunction until these special recordings are viewed on their sets.
Two common faults that suddenly become noticeable when viewing these special recordings are several bright white diagonal lines that run throughout the picture, commonly referred to as RETRACE LINES that can be caused by several different set defects. Also, a pronounced buzzing in the sound can appear.
The retrace lines can be caused of course by a defective part within the vertical blanking circuitry of the television set. They can also be caused by CRT bias controls that have been set too high either inadvertently, or as a routine procedure to extend the usable life of an aging picture tube. Since most of the vertical retrace time is usually at BLACK level, the condition goes totally unnoticed until high levels of white are introduced during the retrace.
As to the buzzing of the sound, this is fairly common to many sets, also due to several defects (including some design), but again, the condition is usually only present for very brief periods such as when some bright lettering is superimposed upon the picture, and as such is generally unnoticed by viewers.
It is not usually noticed until it becomes a more frequent or constant condition as when these special tapes are viewed.
Since the viewing of other tapes or live shows presents no problem, the consumers are being offered differing explanations and solution, many of which lead to an unhappy situation of one kind or another.
The true explanation is far beyond the technical expertise of most lay persons, and a consumer being told to repair an otherwise working television set, or to buy a new one is far from an ideal solution.
One good solution would be the ability to economically remove the offending signal, whether its caused by damaged, scratched, or untrackable video tapes, or is caused by intentionally added pulses, and that is the essence of another embodiment of the present invention.